Process for regenerating spent heavy hydrocarbon hydroprocessing catalyst

ABSTRACT

The present invention relates to a spent hydroprocessing catalyst regeneration process wherein the catalyst is subjected to an initial partial decoking step, followed by impregnation with a Group IIA metal-containing component, and then subjected to a final decoking step.

BACKGROUND OF THE INVENTION

The United States and Canada generate about 100,000,000 pounds of spentbase-metal catalyst per year, about half of which is spenthydroprocessing catalysts. The present invention relates to a processfor regenerating spent heavy hydrocarbon hydroprocessing catalysts. Morespecifically, the present invention relates to a process forregenerating spent heavy hydrocarbon hydroprocessing catalysts that havebeen deactivated with coke and metal deactivants such as nickel andvanadium.

With respect to the present invention, the term hydroprocessing is usedto refer to a process for hydrodemetallation, hydrodesulfurization,hydrodenitrogenation, and hydroconversion wherein the termhydroconversion encompasses the hydrocracking and hydrotreating ofhydrocarbon streams containing asphaltenes and contaminant metals.Hydroprocessing catalysts used to treat heavy hydrocarbon streams, suchas resids, are deactivated as a result of metals deposition and cokedeposition. These deposition materials modify the rate of reaction aswell as accelerate the rate of catalyst deactivation. The various metaldeposits tend to occlude catalyst pores and poison the hydroprocessingcatalyst, while coke deposits similarly reduce pore size and surfacearea of the hydroprocessing catalyst.

Typically, hydroprocessing catalysts possess substantial macroporevolume in order to effect metals removal from the heavy hydrocarbon feedstreams. Heavy hydrocarbon hydroprocessing catalysts possess thecapacity to adsorb contaminant metals, such as nickel and vanadium, inan amount ranging up to about 100 wt. % of the fresh catalyst weight.However, due to the rapid coke deposition rate, the catalyst isdeactivated prior to achieving its full metals adsorption capacity. Suchcatalysts are taken out of service when they contain as little as 10 wt.% nickel plus vanadium. If the spent catalyst is not regenerated, it issubsequently sent to a metals reclamation facility where the proceedstherefrom, in part, depend upon the vanadium content of the spentcatalyst.

Thus, the prior art is replete with processes suitable for regeneratingor rejuvenating such hydroprocessing catalysts. In general, theseprocesses involve removing the deposited contaminant metals, preceded orfollowed by a coke burn-off step. The metals can be removed first, forexample, by acid-leaching with oxalic acid or sulfuric acid, followed bythe decoking step. While some processes afford the extraction of nickeland vanadium without removing active metals [U.S. Pat. No. 4,677,085(Nevitt)], catalytic metals such as cobalt and molybdenum may have to bereimpregnated (Silbernagel, B. G., R. R. Mohan, and G. H. Singhal, "NMRStudies of Metal Deposition on Hydroprocessing Catalysts and Removalwith Heteropolyacids," ACS Div. Ind. Eng. Chem. Catal. Mater.Relationship Struct. Reactivity Symposium (San Francisco 6/13-16/83) ACSSymp. Ser. 248, 91 (1984)).

In the fluidized catalytic cracking (FCC) art, feedstocks containingvanadium are handled by the use of passivation agents. For instance,U.S. Pat. No. 4,451,355 (Mitchell et al.) discloses the use of calcium,antimony, tin, barium, manganese, and bismuth additives to mitigate thepoisonous effects of nickel, vanadium and iron contained in FCCfeedstocks. U.S. Pat. No. 4,364,847 (Tu) discloses the use of lithium tocarry out the subject pacification.

Similarly, U.S. Pat. No. 4,549,958 (Beck et al.) discloses treatment ofhydrocarbon oil having a significant content of vanadium. A fluidizablesorbent is used to demetallize and decarbonize the hydrocarbon oil. Thesorbent contains additive metal components in an amount sufficient tocomplex with and immobilize the flow characteristics of sodium vanadatesor vanadium pentoxide formed during the sorbent oxidative regenerationstep. These additive metals are selected from the group consisting ofMg, Ca, Ba, Sc, Y, La, Ti, Zr, Hf, B, Ta, Mn, In, Te, an element in thelanthanide or actinide series, or an organo-metallic compound of theadditive metal component.

As mentioned above, the metals, especially vanadium, tend to deactivatehydroprocessing catalysts; however, these metals also tend to affect thecatalyst's physical properties, such as the crush strength and theattrition rate of a regenerated hydroprocessing catalyst. Further, mostregeneration processes achieve only partial or mixed restoration offresh activities regardless of whether the contaminant metals areremoved. In this connection, the paper "Studies of Poisoning andRegeneration of Hydrodesulfurization and Hydrodemetallization Catalystduring Treatment of Venezuelan Crude Oils" J. Japan Petrol. Inst.22,(4), 234-242 (1979) shows that where catalysts have been used inhydrodesulfurization (HDS) and hydrodenitrogenation (HDN) processes totreat Venezuelan feeds, the activity loss could only be regenerated byabout 70 to 90 percent because vanadium deposits modified the activitystrongly by blocking catalyst pores and active centers.

In ACS Div. of Petroleum Preprints Vol.27 No.3 679-81 (Sept. 1982), aprocess is disclosed wherein a commercial Al-Co-Mo catalyst isregenerated using a solvent-extraction treatment to extract contaminantmetals with organic reagents capable of forming water-soluble metalcomplexes. The extraction step was followed by a coke burning step. Theregenerated catalyst possessed a lower hydrodesulfurization activity yeta higher hydrodevanadization activity.

U.S. Pat. No. 4,795,726 (Schaper et al.) discloses a method forregenerating spent alumina-based catalysts that have been employed intreating metals-contaminated hydrocarbon feedstocks. The subject processinvolves a steam treatment step, coke burn-off step, and a basic mediumtreatment step. The process may require the addition of catalytic metalsto the regenerated catalyst since the catalytic metals are removedtogether with the vanadium and nickel.

In a paper, Ernst W. R., et al. "GTRC Process For Removing InorganicImpurities From Spent Hydrodesulfurization Catalysts" Minerals andMetallurgical Processing, 4 (2), 78 (1987), a method is disclosed forremoving nickel and vanadium contaminants from spenthydrodesulfurization catalysts that involves pretreating the catalystwith H₂ S followed by the extraction of nickel and vanadium with anacidic solution of ferric ion. The subject method results in the removalof some catalytic metals, such as 50 percent of the cobalt and 5 percentof the molybdenum. The regenerated catalyst possessedhydrodesulfurization (HDS) and hydrodenitrogenation (HDN) activities ofabout 70 percent of the fresh catalyst activities, while thedeactivation rates, for HDS but not HDN of the regenerated catalyst weresuperior to those of the fresh. The regenerated catalyst possessedsuperior hydrodemetallation activity.

Various additives or reagents have been employed to assist in theregeneration of hydroprocessing or hydrorefining catalysts. Forinstance, U.S. Pat. No. 4,581,129 (Miller et al.) discloses a processfor regenerating a hydrorefining catalyst which results in restorationof catalytic activity, no loss in the strength of the support material,and no unacceptable loss of active metals. The subject patent disclosesan embodiment of the invention wherein a mild heat treatment or apartial decoking step is preceded or immediately followed by anextraction of the vanadium and nickel metal contaminants with an acidicsolution. A preferred metals-extraction method is disclosed in U.S. Pat.No. 4,089,806 (Farrell et al.) wherein oxalic acid or in one or morewater-soluble, nitrate-containing compounds, such as nitric acid, andwater-soluble inorganic nitric salts are used. The subject regenerationprocess involves incorporating a phosphorous component after the partialdecoking and extraction step prior to combusting essentially theremainder of the coke from the catalyst.

The subject patent offers the explanation that carrying out the decokingin the absence of phosphorous by combustion releases SO₂ from the sulfuron the catalyst, which SO₂, in the presence of O₂, and the largequantities of vanadium contaminants on the catalyst, are partiallyconverted to SO₃. During combustion, the SO₃ reacts with the aluminacomponent of the hydrorefining catalyst to form aluminum sulfate, and,as a result, the crushing strength, pore volume, surface area, andactivity of the catalyst are often reduced. The subject patent furtherexplains that vanadium on the deactivated catalyst is initially in the+3 or +4 oxidation state in such forms as V₂ S₃ or VS₂. Thus, when asufficient temperature threshold is surpassed, the vanadium is convertedto the +5 oxidation state suitable for promoting the SO₂ conversion toSO₃. Incorporation of phosphorous components with the deactivatedcatalyst is thought to passivate or inhibit, by some chemical reactionmechanism, the vanadium conversion to the +5 oxidation state and therebyinhibit the sulfation mechanism.

The Patentees also point that their metals-extraction process may resultin a reduction of MoO₃ and CoO catalytic components, e.g., from 12 and 4wt. % to 8 and 3 wt. %, respectively. Thus, Patentees suggest in ahighly preferred embodiment of their invention, that catalyticcomponents be reintroduced to the rejuvenated catalyst. Patentees alsosuggest that the rejuvenated catalyst be crushed and reformulated intoparticulate form by extruding a mixture of a gel and the crushedrejuvenated catalyst.

U.S. Pat. No. 4,089,806 (Farrell et al.) as mentioned above alsodiscloses a process for removing vanadium and nickel deactivants fromcontaminated hydrodesulfurization catalysts comprising Group VIB and/orGroup VIII active components on refractory oxide supports. In thisprocess, the spent catalyst is contacted with an aqueous regenerantsolution comprising oxalic acid and one or more solublenitrate-containing compounds from the class consisting of nitric acidand water-soluble inorganic nitrate salts. Suitable nitrate saltsinclude sodium nitrate, ammonium nitrate, potassium nitrate, calciumnitrate, magnesium nitrate, copper nitrate, etc., with the preferredsalt being aluminum nitrate. This contacting results in the removal ofvanadium and nickel contaminants from the surface of the deactivatedcatalyst and substantially rejuvenates the catalyst forhydrodesulfurization purposes, provided that such removal isaccomplished prior to the burning off of any coke present in thecatalyst.

The patent further maintains that subsequent decoking after thecontacting treatment with the regeneration solution is optional.Sufficient activity is restored by the method described without decokingbeing necessary. Patentees maintain that decoking of heavily deactivatedcatalyst may actually result in a loss of some of the activity restoredby the treatment with the regenerant solution. Patentees furthermaintain that it is a critical aspect of the invention that thedeactivated catalyst should not be decoked prior to treatment withregenerant solution, primarily because such decoking is generallycounterproductive. Decoking by combustion prior to the treatmentdescribed herein releases SO₂ from the sulfur in the coke which, in thepresence of O₂ and large quantities of vanadium, deactivates thecatalyst. The so-produced SO₃ then reacts with the alumina catalystsupport to form aluminum sulfate, thereby lowering the crush strength.

Farrell et al. maintain that the rejuvenated catalyst will have at least30 percent, usually 70 percent and an occasion over 80 percent of thefresh original activity. Patentees explain that full restoration isgenerally not possible since 10 percent or less of the active catalyticcomponents are removed from the catalyst during the regenerationprocess.

Finally, U.S. Pat. No. 4,870,044 (Kukes et al.), while not disclosing acatalyst regeneration process, discloses a method for retaining thecrushing strength of hydroprocessing catalysts, wherein hydroprocessingincludes hydrodenitrogenation, hydrodesulfurization andhydrodemetallization of heavy oils, by treating the catalyst with adissolved magnesium compound. The solute in the impregnating solutioncan be any magnesium compound that is at least partially soluble inwater or mixtures of two or more of these magnesium compounds.Non-limiting examples of suitable magnesium compounds are Mg(NO₃)₂,Mg(HCO₃)2, Mg(HSO₄)₂, MgSO₄, magnesium acetate, and the like, preferablyMg(NO₃)₂.

Accordingly, the prior art presents a dilemma in that contaminant metalsmust be removed to retain or restore physical and catalytic propertiesof the spent hydroprocessing catalyst, yet its reclamation value isdiminished when the metal, i.e., vanadium, content of the spent catalystis reduced. Thus, there is a need for a catalyst regeneration processwherein the catalyst poisoning contaminant metals, such as vanadium,need not be removed such that the demetallization capacity of thehydroprocessing catalyst is entirely utilized thereby increasing itseventual reclamation value. Further, there is a need for a regenerationprocess that restores all catalytic activities without the need toreimpregnate catalytic metals while concomitantly maintaining requisitephysical properties such as attrition resistance, attrition resistancebeing an especially important property when the catalyst is employed inan ebullated bed reactor system.

It has now been discovered that when a Group IIA metal component isincorporated into the spent catalyst in accordance with the presentinvention, catalyst activities can be restored with no need for removalof contaminant metals coupled with no loss in catalyst attritionresistance.

SUMMARY OF THE INVENTION

The present invention relates to a process for regenerating a spentheavy hydrocarbon hydroprocessing catalyst. Specifically, the processincludes the steps of carrying out an initial partial decoking stepwherein the spent catalyst is contacted with an oxygen-containing gas atabout 400° F. to about 700° F. The partially decoked catalyst is thenimpregnated with a Group IIA metal-containing component in an amountsuch that the partially decoked catalyst contains about 0.1 to about20.0 wt. % of the Group IIA metal calculated as the oxide. Theimpregnated catalyst is then subjected to another final decoking step inthe presence of an oxygen-containing gas at about 600° F. to about1,400° F.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 through 4 depict plots of devanadation activity, Ramscarbonremoval activity, desulfurization activity, and denitrogenationactivity, respectively, versus weight percentage accumulation of nickelplus vanadium for both comparative and invention catalysts tested in theExample 1.

FIG. 5 depicts a plot of dry attrition loss, weight percent per day,versus regeneration temperature for comparative catalysts and forinvention catalysts.

FIG. 6 depicts a plot of relative BET surface area versus regenerationtemperature, °F. for comparative catalysts and for invention catalysts.

FIG. 7 is a plot of the relative pore volume distributions forcomparative catalysts and invention catalysts.

DETAILED DESCRIPTION OF THE INVENTION

Broadly, the present invention is directed to the regeneration ofcatalysts used in a process for the hydroprocessing of heavy hydrocarbonfeedstocks which contain asphaltenes, Shell hot filtration solidsprecursors, metals, nitrogen compounds, and sulfur compounds. As is wellknown, these feedstocks contain nickel, vanadium, and asphaltenes, e.g.,about 40 ppm up to more than 1,000 ppm for the combined total amount ofnickel and vanadium and up to about 25 wt. % asphaltenes. Thesecatalysts have been used in processes that treat feedstocks with asubstantial amount of metals containing 150 ppm or more of nickel andvanadium and having a sulfur content in the range of about 1 wt. % toabout 10 wt. %. These catalysts have been used to treat feedstocks thatcontain a substantial amount of components that boil appreciably above1,000° F. Examples of typical feedstocks are crude oils, topped crudeoils, petroleum hydrocarbon residua, both atmospheric and vacuumresidua, oils obtained from tar sands and residua derived from tar sandoil, and hydrocarbon streams derived from coal. Such hydrocarbon streamscontain organo-metallic contaminants which create deleterious effects invarious refining processes that employ catalysts in the conversion ofthe particular hydrocarbon stream being treated. The metalliccontaminants found in such feedstocks include, but are not limited to,iron, vanadium, and nickel.

Nickel is present in the form of soluble organo-metallic compounds inmost crude oils and residuum fractions. The presence of nickel porphyrincomplexes and other organo-nickel complexes causes severe difficultiesin the refining and utilization of heavy hydrocarbon fractions, even ifthe concentration of such complexes is relatively small. It is knownthat a cracking catalyst deteriorates rapidly and that its selectivitychanges when in the presence of an appreciable quantity of theorgano-nickel compounds. An appreciable quantity of such organo-nickelcompounds in feedstocks that are being hydrotreated or hydrocrackedharmfully affects such processes. The catalyst becomes deactivated, andplugging or increasing of the pressure drop in a fixed-bed reactorresults from the deposition of nickel compounds in the intersticesbetween catalyst particles.

Iron-containing compounds and vanadium-containing compounds are presentin practically all crude oils that are associated with the highConradson carbon asphaltenic and/or asphaltenic portion of the crude. Ofcourse, such metals are concentrated in the residual bottoms when acrude is topped to remove those fractions that boil below about 450° F.to 600° F. If such residuum is treated by additional processes, such asfluidized catalytic cracking, the presence of such metals as well assulfur and nitrogen adversely affects the catalysts in such processes.It should be pointed out that the nickel-containing compoundsdeleteriously affect cracking catalysts to a greater extent than doiron-containing compounds. If an oil containing such metals is used as afuel, the metals will cause poor fuel oil performance in industrialfurnaces since they corrode the metal surfaces of the furnaces.

While metallic contaminants, such as vanadium, nickel, and iron, areoften present in various hydrocarbon streams, other metals are alsopresent in a particular hydrocarbon stream. Such metals exist as theoxides or sulfides of the particular metal, or as a soluble salt of theparticular metal, or as high molecular weight organo-metallic compounds,including metal naphthenates and metal porphyrins and derivativesthereof.

It is widely known that various organometallic compounds and asphaltenesare present in petroleum crude oils and other heavy petroleumhydrocarbon streams, such as petroleum hydrocarbon residua, hydrocarbonstreams derived from tar sands, and hydrocarbon streams derived fromcoals. The most common metals found in such hydrocarbon streams arenickel, vanadium, and iron. Such metals are very harmful to variouspetroleum refining operations, such as hydrocracking,hydrodesulfurization, and catalytic cracking. The metals and asphaltenescause interstitial plugging of the catalyst bed and reduced catalystlife. The various metal deposits on a catalyst tend to poison ordeactivate the catalyst. Moreover, the asphaltenes tend to reduce thesusceptibility of the hydrocarbons to desulfurization. If a catalyst,such as a desulfurization catalyst or a fluidized cracking catalyst, isexposed to a hydrocarbon fraction that contains metals and asphaltenes,the catalyst will become deactivated rapidly and will be subject topremature replacement.

Although processes for the hydroprocessing of heavy hydrocarbon streams,including but not limited to heavy crudes, reduced crudes, and petroleumhydrocarbon residua, are known, the use of fixed-bed catalytic processesto convert such feedstocks without appreciable asphaltene precipitationand reactor plugging and with effective removal of metals and othercontaminants, such as sulfur compounds and nitrogen compounds, is notcommon because the catalysts employed have not generally been capable ofmaintaining activity and performance.

Thus, the subject hydroprocessing processes are most effectively carriedout in an ebullated bed system. In an ebullated bed, preheated hydrogenand resid enter the bottom of a reactor wherein the upward flow of residplus an internal recycle suspend the catalyst particles in the liquidphase. Recent developments involved the use of a powdered catalyst whichcan be suspended without the need for a liquid recycle. In this system,part of the catalyst is continuously or intermittently removed in aseries of cyclones and fresh catalyst is added to maintain activity.Roughly about 1 wt. % of the catalyst inventory is replaced each day inan ebullated bed system. Thus, the overall system activity is theweighted average activity of catalyst varying from fresh to very old,i.e., deactivated.

Typically, the subject hydroprocessing process is carried out in aseries of ebullated bed reactors. As previously elucidated, an ebullatedbed is one in which the solid catalyst particles are kept in randommotion by the upward flow of liquid and gas. This random motion makesthe attrition resistance of the catalyst a very important property.Attrition resistance is also an important property with respect tocatalysts employed in a fixed bed reactor, because the catalyst mustmaintain its physical integrity, i.e., resist attrition, while it isbeing loaded into a reactor.

In any event, an ebullated bed typically has a gross volume of at least10 percent greater and up to 70 percent greater than the solids thereofin a settled state. The required ebullation of the catalyst particles ismaintained by introducing the liquid feed, inclusive of recycle, if any,to the reaction zone at linear velocities ranging from about 0.02 toabout 0.4 feet per second and preferably, from about 0.05 to about 0.20feet per second.

The catalyst that is regenerated in accordance with the presentinvention preferably contains a hydrogenation component. Preferredhydrogenation components are selected from the group consisting of GroupVIB metals and Group VIII metals.

The addition of a Group VIII metal to the catalyst is especially usefulwhen ebullated bed reactors are employed. In a fixed-bed reactor theactivity of the catalyst dissipates over time, whereas in the ebullatedbed reactor, since fresh amounts of catalyst are continuously orintermittently added, the Group VIII metal provides increased overallaverage activity since the presence of a Group VIII promoter provides ahigher initial activity than the catalyst not containing such apromoter. The freshly added higher, initial activity catalyst isincluded in the weighted average used to determine overall averageactivity. It has been discovered that relatively small amounts of cobaltpresent in a hydroprocessing catalyst provide excellent hydroprocessingactivity in an ebullated bed system. This low cobalt-containinghydroprocessing catalyst is disclosed and claimed in U.S. Pat. No.4,657,665 (Beaton et al.). This low cobalt-containing catalyst also hasa Group VIB metal present in an amount ranging from about 3.5 to about5.0 wt. % calculated as an oxide and based on total catalyst weight. Thecobalt is present in an amount ranging from about 0.4 to about 0.8 wt. %calculated as an oxide (CoO) and based on total catalyst weight.

In any event, the hydrogenation metals can be deposed on a porousinorganic oxide support such as alumina, aluminum phosphate, or aluminumsilicates. Suitably, the composition of the hydroprocessing catalyst ofthe present invention comprises from about 3.0 to about 15.0 wt. % ofthe Group VIB metal, calculated as the oxide. Preferably, the Group VIBmetal is molybdenum. The Group VIB and Group VIII classifications of thePeriodic Table of Elements can be found on page 628 of Webster's SeventhNew Collegiate Dictionary, G. & C. Merriam Company, Springfield, Mass.,U.S.A. (1965). While calculated as the oxide, the hydrogenation metalcomponents of the catalyst can be present as the element, as an oxidethereof, as a sulfide thereof, or mixtures thereof. Molybdenum, which isgenerally superior to chromium and tungsten in demetallation anddesulfurization activity as mentioned above, is the preferred Group VIBmetal component in the demetallation catalyst.

The Group VIII metal can be present in an amount ranging from about 0.4to about 4.0 wt. % calculated as an oxide and based on total catalystweight. The preferred Group VIII metals are cobalt and nickel. Thecobalt and nickel are preferably present in an amount such that the CoOor NiO to Group VIB metal oxide weight ratio varies from about 0.2 toabout 0.3.

The hydroprocessing catalyst regenerated in accordance with the processof the present invention can be prepared by the typical commercialmethod of impregnating a large-pore, high-surface area, inorganic oxidesupport or any other method known to those skilled in the art.Appropriate commercially available alumina, preferably calcined at about800°-1,600° F. (426°-872° C.), for about 0.5 to about 10 hours, can beimpregnated to provide a suitable surface area ranging from about 75 m²/g to about 400 m² /g and a total pore volume within the range of about0.5 cc/g to about 1.5 cc/g.

Preferably, the surface area ranges from about 150 m² /g to about 350 m²/g, a total pore volume of about 0.8 cc/g to about 1.2 cc/g. Thecatalysts most suitably regenerated in accordance with the presentinvention contain pore volume of pores having a diameter greater than1,200 Angstroms of at least 0.05 cc/g, preferably at least 0.1 cc/g, andoptimally from about 0.15 to about 0.3 cc/g.

The porous refractory inorganic oxide, e.g., alumina can be impregnatedwith a solution, usually aqueous, containing a heat-decomposablecompound of the metal to be placed on the catalyst, drying, andcalcining the impregnated material. If the impregnation is to beperformed with more than one solution, it is understood that the metalsmay be applied in any order. The drying can be conducted in air at atemperature of about 80° F. (27° C.) to about 350° F. (177° C.) for aperiod of 0.1 to 5.0 hours. Typically, the calcination can be carriedout at a temperature of about 800° F. (426° C.) to about 1,200° F. (648°C.) for a period of from 0.5 to 16 hours.

Alternatively, the inorganic oxide support can be prepared by mixing asol, hydrosol, or hydrogel of the inorganic oxide with a gelling medium,such as ammonium hydroxide followed by constant stirring to produce agel which is subsequently dried, pelleted, or extruded, and calcined.The hydrogenation metal(s) can then be incorporated into the support asdescribed above or incorporated during the gelling step.

While the hydroprocessing catalyst regenerated in accordance with thepresent invention can be present in the form of pellets, spheres, orextrudates, other shapes are also contemplated, such as a clover-leafshape, cross-shape, or C-shape as disclosed in U.S. Pat. Nos. 3,674,680and 3,764,565 (Hoekstra, et al.).

As mentioned above, during use of the above-described catalysts, theireffectiveness is diminished because hydrocarbon residues in the form ofcoke and contaminant metals such as nickel and vanadium deposit andbuild up on catalyst surfaces and within catalyst pores. The term"contaminant metals" is used to designate metals incidentally compositedwith the catalyst. In some cases residues of the same metal which isused as a catalytic metal deposit incidentally as a contaminant metal onthe catalyst. Nickel is an example of a metal which may be both acatalytic metal and a contaminant metal. In this case the metalintentionally put on the catalyst as a catalytic metal remains"catalytic metal," and the same-metal residue incidentally depositedduring use of the catalyst becomes a "contaminant metal."

In any event, coke and contaminant metals build-up reduces catalystactivity and selectivity, thereby resulting in deactivated, or spentcatalyst. Catalyst activity is a measure of the catalyst's ability toassist the conversion of reactants into products at a specified severitylevel, where severity level means the reaction conditions used, that isthe temperature, pressure, contact time, and presence of diluents, ifany. Catalyst selectivity is a measure of the catalyst's ability to helpproduce a high amount of desired products relative to the amount ofreactants charged or converted.

The spent heavy hydroprocessing catalyst suitable for use in the presentregeneration invention usually contains a total contaminant metalsbuild-up of greater than about 4 wt. % nickel plus vanadium. Thecarbonaceous or coke build-up ranges from about 20 to about 60 wt. %based on total fresh catalyst weight. The relative amounts of metals andcoke present in deactivated catalysts is dependent upon the relativeupstream or downstream position of the catalyst in the reactor while itis in use. The upstream catalyst would contain more metals while thedownstream catalyst would contain more coke.

Prior to carrying out the process of the invention, the catalyst may beoptionally washed with an organic solvent such as toluene or strippedwith an inert gas to remove surface oils.

In accordance with the process of the present invention spent heavyhydroprocessing catalyst is initially subjected to a partial decoking oroxidation step wherein the catalyst is contacted with anoxygen-containing gas at about 400° F. to about 700° F. This partialdecoking step is carried out until up to about 70 weight percent of thedeposited carbonaceous material is removed. The theory behind thispartial decoking step which is not a limitation on the present inventionis that the macropores are sufficiently opened by coke burn-off withoutoxidizing the vanadium contaminants. The preferred gas comprises air andrecycled combustion gases.

Subsequent to the initial decoking step, the partially decoked spentcatalyst is subjected to an impregnation step. The impregnation step iscarried out in a manner known to those skilled in the art. A Group IIAmetal salt impregnation solution is contacted with the partially decokedspent catalyst. The impregnation solution contains a sufficient amountof Group IIA metal such that the impregnated spent catalyst containsfrom about 0.1 to about 20 wt. % of the Group IIA metal calculated asthe oxide and based on fresh catalyst weight.

Preferably, the impregnating solution contains a sufficient amount ofGroup IIA metal to result in an impregnated spent catalyst containing0.5 to 5.0 wt. % Group IIA metal calculated as the oxide and based onfresh catalyst weight. Most preferably or optimally the subject catalystcontains from about 0.5 to 3.0 wt. % Group IIA metal on theabove-described basis.

The preferred Group IIA metals are calcium and magnesium. The Group IIAmetals can be impregnated with aqueous impregnating solutions thatcontain the Group IIA metals as the salt of nitrate, sulfate, sulfite,acetate, benzoate, halides, carbonate, oxy-halides, hydroxide, oxalateand thiosulfates. The impregnation can also be performed using organicsolvents (alcohols, ketones and esters) containing the above-mentionedcompounds.

While not wishing to be bound by theory, it is speculated that thecatalyst-softening or high attrition rate that occurs after conventionalregeneration processes can be attributed to the following mechanism.

When hydroprocessing catalysts are subjected to a combustion step in airunder conditions to remove coke, i.e., 900° to 1000° F., one of thespecies oxidized is the V₃ S₄ sulfide, the predominant vanadium phasedeposited under typical heavy hydrocarbon hydroprocessing conditions.The sulfide is then converted to an equilibrium mixture consisting ofthe V₂ O₅ pentoxide (penta-valent vanadium) as well as sub-oxides andoxy-sulfates (tetra-valent vanadium and lower).

Water formed during the combustion step reacts with the pentoxide toform vanadic acid, VO(OH)₃, a volatile and highly reactive species thatreacts with metals present in the catalyst such as, iron, nickel,aluminum or molybdenum to form mixed metal vanadates. These vanadatescause loss of both catalyst surface area and attrition resistance. Thisprocess is accelerated as the temperature approaches the relatively lowmelting point of the V₂ O₅ pentoxide, which is 1250° F.

Sulfation of the alumina support by sulfur trioxide formed during thecoke-burn has also been proposed as a mechanism leading tocatalyst-softening during regeneration. Vanadium pentoxide, an activeoxidation catalyst, is formed during regeneration and could catalyze theformation of sulfur trioxide from the dioxide, enhancing the aluminasulfation process.

In any event, it is postulated that when a Group IIA metal is added tothe catalyst in accordance with the present invention, prior to thefinal coke-combustion step the Group IIA metal immobilizes vanadiumpentoxide formed during regeneration. This does two beneficial things:

1. It reduces the activity of V₂ O₅ as an SO₂ -oxidation catalyst, thuspreventing formation of aluminum sulfate.

2. It limits the ability of V₂ O₅ to flux on the catalyst surface athigh decoking temperatures, thus preserving catalyst pore structure.

The above-impregnated spent catalyst is then subjected to another finaldecoking or oxidation step wherein the catalyst is contacted with anoxygen-containing gas as in the initial coke-burning step at abourt 600°F. to about 1,400° F. Preferably, the upper temperature limit is about1200° F. and most preferably, the upper temperature limit is about 1000°F. This step is carried out until about 95 percent or more of the cokeis removed from the catalyst.

The regenerated catalyst can then be placed back in service in ahydroprocessing process as described above. Preferably, the catalyst isintroduced into the most upstream portion of a series of ebullated bedreactors.

The operating conditions for the hydroprocessing of heavy hydrocarbonstreams, such as petroleum hydrocarbon residua and the like, comprise ahydrogen partial pressure within the range of about 1,000 psia (68 atm)to about 3,000 psia (204 atm), an average catalyst bed temperaturewithin the range of about 700° F. (371° C.) to about 850° F. (454° C.),a liquid hourly space velocity (LHSV) within the range of about 0.1volume of hydrocarbon per hour per volume of catalyst to about 5 volumesof hydrocarbon per hour per volume of catalyst, and a hydrogen recyclerate or hydrogen addition rate within the range of about 2,000 standardcubic feet per barrel (SCFB) (356 m³ /m³) to about 15,000 SCFB (2,671 m³/m³). Preferably, the operating conditions comprise a hydrogen partialpressure within the range of about 1,200 psia to about 2,800 psia(81-190 atm); an average catalyst bed temperature within the range ofabout 730° F. (387° C.) to about 820° F. (437° C.); and a LHSV withinthe range of about 0.15 to about 2; and a hydrogen recycle rate orhydrogen addition rate within the range of about 2,500 SCFB (445 m³ /m³)to about 5,000 SCFB (890 m³ /m³).

If the regenerated catalyst of the present invention were to be used totreat hydrocarbon distillates, the operating conditions would comprise ahydrogen partial pressure within the range of about 200 psia (13 atm) toabout 3,000 psia (204 atm); an average catalyst bed temperature withinthe range of about 600° F. (315° C.) to about 800° F. (426° C.); a LHSVwithin the range of about 0.4 volume of hydrocarbon per hour per volumeof catalyst to about 6 volumes of hydrocarbon recycle rate or hydrogenaddition rate within the range of about 1,000 SCFB (178 m³ /m³) to about10,000 SCFB (1,381 m³ /m³). Preferred operating conditions for thehydroprocessing of hydrocarbon distillates comprise a hydrogen partialpressure within the range of about 200 psia (13 atmos) to about 1,200psia (81 atmos); an average catalyst bed temperature within the range ofabout 600° F. (315° C.) to about 750° F. (398° C.); a LHSV within therange of about 0.5 volume of hydrocarbon per hour per volume of catalystto about 4 volumes of hydrocarbon per hour per volume of catalyst; and ahydrogen recycle rate or hydrogen addition rate within the range ofabout 1,000 SCFB (178 m³ /m³) to about 6,000 SCFB (1,068 m³ /m³).Generally, the process temperatures and space velocities are selected sothat at least 30 vol. % of the feed fraction boiling above 1,000° F. isconverted to a product boiling below 1,000° F. and more preferably sothat at least 60 vol. % of the subject fraction is converted to aproduct boiling below 1,000° F.

The present invention is described in further detail in connection withthe following examples, it being understood that the same are forpurposes of illustration and not limitation.

EXAMPLE 1

In the present example several tests were carried out to show theimprovement afforded by the invention hydroprocessing catalystregeneration process over comparative prior art regeneration processes.These improvements included a restoration of initial catalyst activityas explained below and the achievement of an acceptable catalystattrition rate.

Table I below sets out the chemical and physical properties forreference catalysts A and B, comparative catalysts C and D, andinvention catalyst E.

                  TABLE I                                                         ______________________________________                                                      Catalyst                                                                        A        B          C                                                                             Decoked                                                            Equilibrium                                                                              900° F.                            Chemical Analyses                                                                             Fresh    Spent      Com-                                      (wt %, Fresh Basis, Al-Tie)                                                                   Control  Control    parative                                  ______________________________________                                        Ni              --       1.64                                                 V               --       6.09                                                 Fe              --       1.15                                                 Na.sub.2 O      0.032    0.26                                                 SiO.sub.2       0.035    0.21                                                 CoO             3.57     3.51                                                 MoO.sub.3       14.3     15.4                                                 Mg              --       --         0.07                                      C               --       54.10      <0.1                                      H               --       2.51       0.30                                      S               0.12     8.02       2.52                                      Al (Fresh Basis)                                                                              (43.6)   (43.6)     (43.6)                                    Al (Decoked Basis)                                                                            --       --         30.8                                      Al (Spent Basis)                                                                              --       25.0       --                                        Physical Inspections                                                          (Fresh Basis, Al-Tie Point)                                                   Dry LOA, wt % Fines/Day                                                                       1.78     --         4.50,4.92                                 N.sub.2 Desorption                                                            BET, m.sup.2 /g 311      18.5       282                                       BJH, 1200 Å-, cc/g                                                                        0.635    0.073      0.653                                     2500, 1200 Å-, cc/g                                                                       0.760    0.091      0.767                                     Hg Porosimetry  0.22     0.21       0.33                                      PV, 1200 Å+, cc/g                                                         ______________________________________                                                       Catalyst                                                                        D (1)      E                                                                  Decoked    Mg +  Decoked                                     Chemical Analyses                                                                              900° F.                                                                           900° F.                                    (wt %, Fresh Basis, Al-Tie)                                                                    Comparative                                                                              Invention                                         ______________________________________                                        Ni                                                                            Fe                                                                            Na.sub.2 O                                                                    SiO.sub.2                                                                     CoO                                                                           MoO.sub.3                                                                     Mg               1.06                                                         C                <0.1                                                         H                0.22                                                         S                1.93                                                         Al (Fresh Basis) (43.6)                                                       Al (Decoked Basis)                                                                             33.6                                                         Al (Spent Basis) --                                                           Physical Inspections                                                          (Fresh Basis, Al-Tie Point)                                                   Dry LOA, wt % Fines/Day                                                                        4.41,6.10  2.44                                              N.sub.2 Desorption                                                            BET, m.sup.2 /g             263                                               BJH, 1200 Å-, cc/g      0.600                                             2500, 1200 Å-, cc/g     0.705                                             Hg Porosimetry              0.26                                              PV, 1200 Å+, cc/g                                                         ______________________________________                                         (1) Catalyst C and D have similar properties                             

The attrition rates set forth above in Table I were all measured inaccordance with the following procedure. For each catalyst sample, about100 g thereof were passed over a U.S. 30 mesh sieve to remove fines.Each sample was then calcined at 1,000° F. for about one hour. Eachsample was subsequently cooled to room temperature in a desiccator. Eachsample weight, W(b) was then recorded. Each sample was then placed in anabrasion test drum as described in ASTM method D4058, followed by atumbling of the drum at 60 rpm for 22 hours. Each sample was thenremoved from the drum and passed over a U.S. 30 mesh size screen. Eachsample was then recalcined at 1,000° F. for one hour. Each sample wasthen cooled to room temperature and placed in a desiccator. Each samplewas then again weighed W(a) and recorded.

The loss on attrition (LOA) was then calculated for each sample inaccordance with the following formula: ##EQU1##

An acceptable loss on attrition rate is less than about 3.0 wt. %/dayand most preferably less than about 2.5 wt. %/day.

Activity testing was carried out in a fixed-bed upflow reactor atconditions including 780° F., 0.2 LHSV, 0.5 SCFH hydrogen addition rate,and 2,000 psig total pressure. In each test 40 cc of the respectivecatalyst was diluted with 40 cc of alundum chips. The feedstock used ineach test was a vacuum resid whose properties are set out below in TableII.

                  TABLE II                                                        ______________________________________                                        FEED INSPECTION                                                               ______________________________________                                                 Total Liquid                                                                  Gravity, °API                                                                      6.9                                                               1000° F.+, wt %                                                                    85.7                                                              650-1000° F., wt %                                                                 14.3                                                              Ni, ppm     68                                                                V, ppm      288                                                               Fe, ppm     20                                                                S, wt %     4.8                                                               N, wt %     0.504                                                             O, wt %     0.61                                                              C, wt %     83.91                                                             H, wt %     10.26                                                             Ramscarbon, wt %                                                                          19.6                                                              NMR-C.sub.A, atom %                                                                       30.6                                                              1000° F.+                                                              Gravity, °API                                                                      4.0                                                               Ni, ppm     76                                                                V, ppm      319                                                               Fe, ppm     18                                                                S, wt %     5.1                                                               N, wt %     0.620                                                             O, wt %     0.65                                                              Ramscarbon, wt %                                                                          22.0                                                              NMR-C.sub.A, atom %                                                                       --                                                                Oils, wt %  21.4                                                              Resins, wt %                                                                              62.8                                                              Asphaltenes, wt %                                                                         14.8                                                     ______________________________________                                    

FIGS. 1 through 4 depict devanadation activity, Ramscarbon removalactivity, desulfurization activity, and denitrogenation activity,respectively, versus weight percentage accumulation of nickel plusvanadium upon the catalyst for each of the tested catalysts A through E.These figures contain the activity data measured in connection with thetesting of the respective catalysts as described above.

Denitrogenation activity was calculated assuming pseudo-first-orderkinetics with an activation energy of 45,400 Btu/lb-mol in accordancewith the following equation. ##EQU2## where: A_(N) is HDN activity

N_(F) is feed nitrogen content, ppm

N_(P) is product nitrogen content, ppm

LHSV is liquid (volumetric) hourly space velocity, hr⁻¹

K_(N) is pre-exponential feed nitrogen factor=92 hr⁻¹ psig⁻¹

P is total pressure, psig

E is activation energy 45,400 Btu/lb-mol

T is absolute average temperature, °R.,

Devanadation activity was calculated on the basis of the followingfirst-order rate equation, where the activation energy was 83,300Btu/lb-mol: ##EQU3## where A_(v) is devanadation activity

V_(P) is product vanadium content, ppm

V_(F) is feed vanadium content, ppm

K_(v) is pre-exponential feed vanadium factor (2730×10⁶ hr⁻¹ psig⁻¹)

P is total pressure, psig

T is absolute average temperature, °R.,

E is activation energy, 83,300 Btu/lb-mol

LHSV is liquid (volumetric) hourly space velocity, hr⁻¹

Desulfurization activity assuming pseudo-second order with an activationenergy of 83,300 BTU/lb-mol was calculated in accordance with thefollowing equation: ##EQU4## where A_(s) is desulfurization activity

k_(s) is pre-exponential feed sulfur factor (1420×10⁶ hr⁻¹ psig⁻¹ /wt%)

S_(P) is product sulfur content, wt%

P is total pressure, psig

S_(F) is feed sulfur content, wt%

T is absolute average temperature, °R.,

E is activation energy, 83,300 Btu/lb-mol

LHSV is liquid (volumetric) hourly space velocity, hr⁻¹

Ramscarbon removal activity assuming pseudo-second-order kinetics withan activation energy of 83,300 BTU/lb-mol was calculated in accordancewith the following equation: ##EQU5## where A_(r) is ramscarbon removalactivity

k_(r) is pre-exponential feed ramscarbon factor (103×10⁶ hr⁻¹ psig⁻¹/wt%)

R_(P) is product ramscarbon content, wt%

R_(F) is feed ramscarbon content, wt%

P is total pressure, psig

T is absolute average temperature, °R.,

E is activation energy, 83,300 Btu/lb-mol

LHSV is liquid (volumetric) hourly space velocity, hr⁻¹

Catalyst A was a fresh commercial resid hydroprocessing catalyst. FIGS.1 through 4 show that all of the initial activities were high, and TableI shows an acceptable attrition rate for this fresh catalyst.

Catalyst B was a spent commercial resid hydroprocessing catalyst thatwas removed from a commercial resid hydroprocessing unit. The catalystcontained 8.0 wt. % nickel plus vanadium. This catalyst was first washedwith toluene and hexane to strip off excess oil, followed by a dryingstep carried out overnight in a nitrogen-purged convection oven at 250°F. prior to subjecting it to the activity testing. FIGS. 1 through 4clearly show that the spent catalyst possessed activities below thefresh catalyst.

Catalyst C was obtained by using a comparative regeneration process uponthe oil-stripped catalyst B. Oil-stripped catalyst B was subjected to asingle coke-burning step at 900° F. for 2 hours in the presence of air.In particular, oil-stripped catalyst B was placed in a 7×10 inchcalcining basket, spread evenly to a bed depth of one-half inch andplaced in a large muffle furnace (5 ft³ internal volume) in the presenceof a 3.4 SCFH air purge. The temperature was increased from 400° to 900°F. at approximately 150° F. per hour and then held at 900° F. for the 2hours.

Table I shows that catalyst C had an unacceptable attrition rate of 4.5to 5.0 wt. %. The initial activities of catalyst C as shown in FIGS. 1through 4 were about the same as that of fresh catalyst A.

Catalyst D was prepared by subjecting catalyst B to a comparativeregeneration process wherein the spent catalyst was exposed to air at900° F. for 16 hours in a single coke-burning step. Table I shows thatcatalyst D possessed an unacceptable catalyst attrition rate.

Catalyst E was obtained by carrying out the regeneration process inaccordance with the present invention. Spent catalyst B was firstsubjected to a partial decoking step in the presence of air.Specifically, a large muffle furnace was employed wherein thetemperature was increased from 400° F. at 100° F. per hour incrementsand then held at 650° F. for 2 hours. This partial decoking step removedabout 50 percent of the deposited coke and increased the pore volumefrom about 0.1 to about 0.4 cc/g. The partially decoked catalyst wasthen impregnated with an aqueous solution of magnesium nitrate toachieve a level of 1.0 wt. % Mg on the catalyst based on a freshcatalyst basis. The subject catalyst was then dried overnight in anair-purged convection oven. A final decoking or coke-burning step wascarried out in the presence of air at 900° F. for about 2 hours aftercarrying out the above described catalyst C temperature increaseprocedure.

Table I clearly shows that the invention regeneration process affords acatalyst (catalyst E) having an acceptable attrition rate. Further,FIGS. 1 through 4 show that the initial activities are about the same asfor fresh catalyst A. FIG. 1 shows that catalysts A, C, D, and E havehigh and stable devanadation activity corresponding to about 99 percentvanadium removal, while the spent catalyst B has inferior devanadationactivity.

FIGS. 2 through 4 show that the regenerated catalysts all possessed highinitial activities comparable to the fresh catalyst A. The rate ofdeactivation for both the prior art-regenerated and theinvention-regenerated catalysts are equivalent or marginally acceleratedcompared to fresh catalyst A, probably due to the 8 wt. % nickel plusvanadium deposits on the regenerated catalysts.

EXAMPLE 2

FIGS 5-7 present additional data showing that a spent heavy hydrocarbonhydroprocessing catalyst can be regenerated without lowering itsattrition resistance by first impregnating the partially decokedcatalyst with a Group IIA metal component prior to calcination to removeall residual carbon. In the present example, two sets of regeneratedcatalysts were compared:

Several catalyst samples were examined in the present example.Specifically, Catalysts A, C, and invention Catalyst E from Example 1were examined. Further, a control or comparative, catalyst F wasexamined. Catalyst F is Catalyst B from Example 1 regenerated using theprior art regeneration process wherein the coke burn-off was carried outat 1200° F. Invention Catalyst G was prepared in accordance with thepresent invention wherein Catalyst B of Example 1 was regenerated asfollows. Specifically, catalyst B was partially decoked in air at 600°F., then impregnated with an aqueous magnesium nitrate solutionsufficient to add 1 wt. % Mg content (fresh basis) to the catalyst,dried overnight in an air-purged convection oven at 250° F., and thencalcined or decoked in air at 1200° F.

FIG. 5 shows that loss on dry attrition is reduced by up to a factor ofabout three for the invention Catalysts E and G compared to the priorart-regenerated Catalysts C and F at regeneration temperatures of 900°F. and 1200° F., respectively.

FIG. 6 shows that the invention Catalyst G retained 70 percent of itssurface area when the regeneration temperature was increased from 900°to 1200° F. compared to only 50 percent surface area retention for thecomparative Catalyst F. Surface area retention is important formaintaining good catalytic activity, especially if unintentionaltemperature excursions or increases occur in a commercial coke-burningregenerator.

FIG. 7 compares the pre size distributions (PSD) of Catalysts A, C, E, Fand G. In the Figure, "Fresh" designates the PSD curve of the freshCatalyst A. The curves designated "Regen at 900° F." in the Figure showthat the invention Catalyst E and the comparative Catalyst C, whenregenerated at 900° F. have essentially identical mesopore PSD curves,which are similar in shape to the "Fresh" curve. The curves designated"Regen at 1200° F." in the Figure show that the comparative Catalyst Fsintered after regeneration at 1200° F. to form larger size mesopores.The invention Catalyst G, however, did not appear to sinter as much andretained much of the character of the "Fresh" catalyst PSD curve.

The reason regeneration temperatures in the 900° to 1200° F. range wereinvestigated was to simulate expected commercial conditions in aregenerator. At 900° F. or higher, all coke on the spent catalyst can beburned off. However, auto-ignition or poor temperature control in theregenerator catalyst bed may cause temperature "run away" excursions upto as high as 1400° F., which would accelerate the catalyst softeningprocess.

FIGS. 5-7 clearly show that the invention catalysts possess a superiorability to survive the harsh conditions prevailing in a commercialregenerator as compared to the comparative catalysts.

EXAMPLE 3

The present example serves to show the improvement afforded by theinvention regeneration process when calcium is used as the Group IIAmetal.

A spent catalyst whose properties are set out below in Table III wasregenerated in accordance with the invention. The subject catalyst waspartially decoked at 600°0 F., followed by impregnation with a calciumnitrate-containing solution in an amount sufficient to yield 1 wt. % Caon the partially decoked catalyst calculated as the oxide, and based onthe fresh catalyst weight. The catalyst was then fully decoked at 900°F.

                  TABLE III                                                       ______________________________________                                        PROPERTIES OF SPENT (DE-OILED)                                                LC-FINING CATALYST                                                            CHEMICAL ANALYSES (WT. % FRESH BASIS):                                        ______________________________________                                                Ni   1.64                                                                     V    6.09                                                                     Fe   1.15                                                                     Mo   10.3                                                                     Co   2.74                                                                     C    54.10                                                                    H    2.51                                                                     S    8.02                                                             ______________________________________                                    

The regenerated catalyst was then subjected an attrition test asdescribed in Example 1. The invention regenerated catalyst achieved adry loss on attrition of 1.34 wt. %/day, well below the acceptable rateof 3.0 wt. %/day and below the comparative or control regeneratedcatalysts described in Example 1.

What is claimed is:
 1. A process for regenerating a metals contaminatedspent hydrocarbon hydroprocessing catalyst with a total containmentmetals build-up of greater than 4 wt% nickel plus vanadium, based on thetotal weight of fresh catalyst, comprising the steps:(a) partiallydecoking said catalyst in an initial coke-burning step wherein saidcatalyst is contacted with an oxygen-containing gas at a temperatureranging from about 400° F. to about 700° F.; (b) impregnating saidpartially decoked catalyst with a Group IIA metal-containingimpregnation solution such that the impregnated partially decokedcatalyst contains from about 0.1 to about 20.0 wt. % of said Group IIAmetal calculated as the oxide and based on the fresh weight of saidspent catalyst; and (c) decoking said impregnated catalyst in a finalcoke-burning step wherein said impregnated catalyst is contacted with anoxygen-containing gas at a temperature of about 600° F. to about 1400°F.
 2. The process of claim 1 wherein said final coke-burning step iscarried out at a temperature ranging from about 600° F. to about 1200°F.
 3. The process of claim 1 wherein said final coke-burning step iscarried out at a temperature ranging from about 600° F. to about 1000°F.
 4. The process of claim 1 wherein said Group IIA metal is magnesium.5. The process of claim 1 wherein said Group IIA metal is calcium. 6.The process of claim 1 wherein said partially decoked catalyst containsabout 0.5 to about 5.0 wt. % of said Group IIA metal.
 7. The process ofclaim 1 wherein said partially decoked catalyst contains about 0.5 toabout 3.0 wt. % of said Group IIA metal.
 8. The process of claim 1wherein said initial coke-burning step is carried out until up to about70 wt. % of the coke present in said spent catalyst is removed.
 9. Theprocess of claim 1 wherein said spent catalyst prior to beingdeactivated by contact with a hydrocarbon stream possessed at least 0.05cc/g pore volume in pores having pore diameters greater than about 1200Angstroms.
 10. The process of claim 1 wherein said spent catalyst priorto being deactivated by contact with a hydrocarbon stream possessed atleast 0.1 cc/g pore volume in pores having pore diameters greater thanabout 1200 Angstroms.
 11. The process of claim 1 wherein said spentcatalyst prior to being deactivated by contact with a hydrocarbon streampossessed a pore volume of about 0.15 to about 0.3 cc/g in pores havinga diameter greater than about 1200 Angstroms.